We analyzed the results of consecutive medical school interviews at the University of Toronto between 2004 and 2009. … A total of 2926 candidates were interviewed over the 6-year period. … Overall, those interviewed on rainy days received about a 1% lower score than those interviewed on sunny days … The difference in scores was equivalent to about a 10% lower total mark on the Medical College Admission Test.

That is Donald Redelmeier and Simon Baxter in the Canadian Medical Association Journal.

Hat tip to Freakonomics and .Plan.